Back out "Temporarily disable stack traces for libc++ arm64"

Summary:
As with all temporary solutions, this turned out to be a lot less
temporary than I was hoping for :D I was never able to figure out the
original issue, but we've since changed unwinders from libgcc to LLVM's
libunwind, and hopefully that fixes this issue as well.

Original commit changeset: 088c8a781d00

Original Phabricator Diff: D19718354 (501e6df)

Reviewed By: lanza

Differential Revision: D35133990

fbshipit-source-id: 8959891f1a8a7687be5786cb5c00c257cefdef36

cxx/lyra/cxa_throw.cpp

@@ -131,14 +131,11 @@ void trace_destructor(void* exception_obj) {
 
 [[noreturn]] void
 cxa_throw(void* obj, const std::type_info* type, destructor_type destructor) {
-  // TODO(T61689492): Re-enable Lyra stack traces for libc++ arm64
-#if !defined(__aarch64__)
   if (enableBacktraces.load(std::memory_order_relaxed)) {
     std::lock_guard<std::mutex> lock(*get_exception_state_map_mutex());
     get_exception_state_map()->emplace(
         obj, ExceptionState{ExceptionTraceHolder(), destructor});
   }
-#endif
 
   original_cxa_throw(obj, type, trace_destructor);
 }